As is well known, in the electrical engineering sector, the term “rotor” is used to define the drive shaft of an electric motor. The rotor, in a machine with moving parts, is the set of rotating parts situated opposite the stator which is the stationary part.
It is known to construct rotors using sintered materials.
The sintering process consists in compacting and transforming materials reduced to powder form into a unified composition; this treatment is performed at a temperature lower than the melting point of the material.
The characteristic property of a component made by means of sintering is the extreme hardness of the component, together with the corresponding low cost in the case of mass-production.
In this way, cores made of magnetic materials with a high magnetic permanence are obtained. Although sintered materials have a good compressive strength, a limiting drawback consists in the very poor tensile strength; this is due to the nature itself of the sintered materials which are not obtained by means of fusion, but by means of compaction of powders.
In synchronous electric motors, in particular high-speed permanent-magnet motors, the rotor is subject to centrifugal forces (perpendicular to the external surface of the rotor itself) which tend to break the rotor, exerting tensile forces which cannot be withstood, resulting potentially in complete destruction thereof.
Usually these forces are opposed by materials which are suitably wound around the rotor, called banding tapes, which, by limiting expansion of the magnet due to the centrifugal forces, exert a pressure on the magnetic material, so as to prevent destruction thereof.
For such an application in motors characterized by rotational speeds of more than 50,000 rpm, usually a composite material, e.g. carbon fiber, is used (this because of its good non-magnetic, electrical and mechanical properties), said material being wound onto the rotor and compacted using special resins.
Although carbon fiber has good mechanical properties, there is nevertheless a maximum limit of the tensile forces which such a fiber is able to exert on the rotor; the amount of this force varies depending on the diameter of the yarn used to prepare the banding tape.
In fact, considering that the centrifugal force exerted by the magnet on the fiber increases with the square of the angular velocity, it is clear that, with an increase in the latter, the forces to be opposed by the fiber become increasingly greater, until conventional banding systems become inadequate since the elasticity of the banding is such that it cannot withstand the forces of the magnet and therefore is unable to prevent breakages due to the tensile loads of the magnet itself.
The technical problem is to provide a rotor with a banding system which pre-compresses the magnet and does not result in it being subjected to major tensile loads, but only compressive loads.